找不到接受所提供的參數「下標」超載 - 當轉換雨燕1.2至2

Question

我從雨燕1.2轉換項目2.下面的代碼：

func pageControlChanged(pageControl: UIPageControl) { 
    // get the current and upcoming page numbers 
    let currentTutorialPage = (viewControllers[0] as! partyStepViewController).page 
    let upcomingTutorialPage = pageControl.currentPage 

    // what direction are we moving in? 
    let direction: UIPageViewControllerNavigationDirection = upcomingTutorialPage < currentTutorialPage ? .Reverse : .Forward 

    // set the new page, animated! 
    setViewControllers([partyStepForPage(upcomingTutorialPage)], 
     direction: direction, animated: true, completion: nil) 
} 

給我的以下錯誤：

Could not find an overload for 'subscript' that accepts the supplied arguments

對代碼的特定行：

let currentTutorialPage = (viewControllers[0] as! partyStepViewController).page 

這個錯誤是什麼消息呢？我該如何解決它？

這裏有一些鏈接到我的代碼：

• a zipfile of the whole project，不會轉換爲斯威夫特2
• partyStepViewController
• partyPagedViewController

Answer 1

的問題是，viewControllersis now an Optional。

所以，你必須解開它，然後才能下標是：viewControllers![0]